In the process of manufacturing electronic parts members such as LSI chips or the like, the chips may be required to have extremely clean surfaces thereof. In the process of manufacturing LSI chips, for example, an insulator film of material such as SiO.sub.2 is formed on the surface of the silicon wafer, and then a resist layer is formed on the insulator film in the predetermined pattern. The insulator film where the resist layer is not formed is etched to expose the metal silicon underneath said insulator film, and then the surface thereof is cleaned. Then elements of p-type or n-type are introduced according to the intended use, forming embedded metal wiring patterns such as Al patterns (lithography process). Said steps are repeated to complete the LSI chips. At the time of introducing elements of p-type or n-type, or in the process of forming embedded metal wiring patterns on the chips, adhesion of impurities such as particulate foreign materials, metals, organic matter and natural oxide film to the exposed surface of silicon may lead to the degradation of the chip' characteristics due to deficient electrical connection between the metal wiring and the metal silicon and an increase in contact resistance. Therefore, in the process for manufacturing LSI chips, the cleaning step of the surface of the silicon wafer is very important for producing high performance chips, requiring as much removal of impurities adherent to the wafer surface as possible.
Conventionally, the cleaning of the silicon wafer is carried out by the combination of cleaning by a solution such as a sulfuric acid-hydrogen peroxide mixture solution, a hydrochloric acid-hydrogen peroxide mixture solution, a hydrofluoric acid solution, ammonium fluoride solution, and cleaning by high-purity water to remove organic matter, particles, metals or natural oxide films adhered to the surfaces of the silicon wafer without damaging the flatness of the surface of the silicon wafer at atomic level.
A detailed example of the conventional cleaning process of silicon wafers will be described in the steps from (1) to (13) below:
(1) Step of cleaning with sulfuric acid-hydrogen peroxide; cleaning with a mixture solution of sulfuric acid and hydrogen peroxide solution in a volume ratio of 4:1 at 130.degree. C., 10 min. PA0 (2) Step of cleaning with high-purity water; cleaning with high-purity water for 10 min. PA0 (3) Step of cleaning with hydrofluoric acid; cleaning with hydrofluoric acid in a concentration of 0.5% for 1 min. PA0 (4) Step of cleaning with high-purity water; cleaning with high-purity water for 10 min. PA0 (5) Step of cleaning with ammonia-hydrogen peroxide; cleaning with a mixture solution of ammonia water, hydrogen peroxide solution and high-purity water in a volume ratio of 0.05:1:5 at 80.degree. C. for 10 min. PA0 (6) Step of cleaning with high-purity water; cleaning with high-purity water for 10 min. PA0 (7) Step of cleaning with hydrofluoric acid; cleaning with hydrofluoric acid in a concentration of 0.5% for 1 min. PA0 (8) Step of cleaning with high-purity water; cleaning with high-purity water for 10 min. PA0 (9) Step of cleaning with hydrochloric acid-hydrogen peroxide; cleaning with a mixture solution of hydrochloric acid, hydrogen peroxide solution and high-purity water in a volume ratio of 1:1:6 at 80.degree. C. for 10 min. PA0 (10) Step of cleaning with high-purity water; cleaning with high-purity water for 10 min. PA0 (11) Step of cleaning with hydrofluoric acid; cleaning with hydrofluoric acid in a concentration of 0.5% for 1 min. PA0 (12) Step of cleaning with high-purity water; cleaning with high-purity water for 10 min. PA0 (13) Spin dehydration or IPA steam drying
In the above step (1), removing chiefly the organic matter adhered to the surface of the silicon wafer is carried out. In the step (5), mainly to remove the particulate adhered to the surface of the silicon wafer is carried out. In the step (9), mainly to remove metal impurities adhered to the surface of the silicon wafer is carried out. Further, in the steps (3), (7) and (11), to remove the natural oxide film on the surface of the silicon wafer is carried out. Furthermore, each cleaning solution in each step is often capable of removing other contaminants than those principal ones. Since, for example, said sulfuric acid-hydrogen peroxide solution used in the step (1) also has a powerful ability to remove adherent metallic impurities in addition to adherent organic matter, a method is also available for removing a plurality of contaminating objects with a single cleaning solution, in addition to the method for removing a different impurity with each cleaning solution described above.
In the cleaning process of silicon wafers, as a method for contacting cleaning agents or high-purity water to the surface of silicon wafer, a method so-called a batch type cleaning method is frequently employed, wherein a plurality of silicon wafers are soaked batchwise in a cleaning bath containing a cleaning solution or high-purity water. Furthermore, there has been used as a cleaning process while carrying our circulation and filtration of cleaning solution to prevent contamination thereof, and as rinsing processes with high-purity water after treatment with cleaning solution, an overflow rinse system, wherein high-purity water is supplied from the bottom of the cleaning bath and overflowed from the top, and a quick dump rinse system, wherein high-purity water fills the cleaning bath once deep enough to soak the entire surface of wafers and then is dumped from the bottom in a single action. Recently, in addition, there has been used a method for showering the wafer surface with cleaning solutions or high-purity water, and a method for cleaning by spraying cleaning solutions or high-purity water to the center of wafers rotating at high speed, so-called "single wafer rinse".
The cleaning treatment with high-purity water performed after the cleaning with cleaning solutions described above is to rinse remaining cleaning solutions off from the wafer surface. Therefore, high-purity water from which particles, colloidal substances, organic matter, metal ions, anions, a dissolved oxygen gas or the like have been mostly removed to the extreme is used as the rinse water. Such high-purity water is also used as a solvent of cleaning solutions.
Recently, the density of integration of LSI has increased dramatically. The lithography process is now performed as many as 20 to 30 times in the manufacturing process of LSI, although it was done no more than several times in the early stage of LSI. This increase in the number of the lithography processes requires a correspondingly increased frequency of cleaning wafers. Thus, the cost of products is rising with the increasing costs, such as material cost of cleaning solutions and high-purity water and post-treatment cost of used cleaning solutions and used high-purity water, cost of air venting to remove the gas of cleaning solutions from the clean room produced by high temperature cleaning, and the like cost.
In the manufacturing process of LSI as mentioned above, particles adhered to the surface of the silicon wafer reduce the yield of LSI significantly. It is, therefore, particularly important to remove particles from the surface of the wafer. Conventionally, an ammonia-hydrogen peroxide mixture solution has been used to remove the particles adhered to the surface of the silicon wafer. However, ammonia and hydrogen peroxide react with each other to form chemical species other than themselves. The cleaning effect of these chemical species has not been identified. Also, scientific knowledge has not been obtained as to which composition and ratio of ammonia and hydrogen peroxide are most suitable for effective cleaning. Thus, the concentration of ammonia-hydrogen peroxide mixture solution presently used is higher than necessary to remove particles on the surface of the wafer thoroughly. That is, in order to remove particles on the surface of the wafer, the surface may be etched by using some alkaline solution, but in this case, it is necessary to prevent detached particles from being adhering again to the surface of the wafer. To do so, the code of surface potential of the surface of the wafer and particles needed to be made the same in order to repulse each other electrically. To accomplish this, the pH of the cleaning solution had to be raised to get strong alkaline solution. Further, since the use of such strong alkaline solution resulted in roughened surface of the wafer unnecessarily, hydrogen peroxide solution had to be added to the cleaning solution to form oxide film on the surface of the wafer by virtue of hydrogen peroxide, so that the roughened surface of the wafer could be avoided. As a result, there have been problems such as spending a lot of reagents unnecessarily, increased use of high-purity water for rinsing and increased cost for effluent treatment.
Additionally, in said prior art cleaning processes, there is likely to exist the following several problems, when the rinsing with high-purity water follows the cleaning with hydrofluoric acid to remove the natural oxide film formed on the surface of silicon wafers.
First, the high-purity water containing a dissolved oxygen gas of 10 ppm or less is used to prevent the surface of a silicon wafer from being oxidized by a dissolved oxygen gas. However, the oxygen gas in the air dissolves in high-purity water in a moment at a cleaning bath because the cleaning bath is usually not airtight (the concentration of a dissolved oxygen gas may increase to about 100 ppm). The surface of a silicon wafer may be subjected to oxidation by such high-purity water used for cleaning and in particular, in the case of n+ silicon susceptible to oxidation, oxide film of a few angstroms is easily formed on the surface of a silicon wafer.
Second, even though the above mentioned problem associated with the oxygen gas in the air dissolved into high-purity water is solved by making the cleaning bath air-tight, the surface of silicon wafers may be etched by hydroxyl ions, thus roughened surface up to a few angstroms in depth being produced. This is because of hydroxyl ions present in neutral high-purity water in the concentration of 10.sup.-7 mole/l. Furthermore, silicon detached through etching by hydroxyl ions may adhere to the surface of silicon wafers, thus causing foggy surface of silicon wafers.
Moreover, in the cleaning process of LSI or the like as mentioned above, organic matter attached to the surface of the silicon wafer deteriorates the performance of LSI and reduces the yield of LSI significantly. Additionally, the film of organic matter includes inside thereof metal impurities and particles attached thereto and natural oxide film formed. Thus, there has bee a problem in that cleaning with hydrofluoric acid solution, hydrochloric acid-hydrogen peroxide and ammonium hydroxide-hydrogen peroxide mixture solution cannot remove metal impurities, particles and natural oxide film thoroughly.
For this reason, the conventional cleaning process is usually designed to remove organic matter first. As shown in the example of cleaning process mentioned earlier, the conventional cleaning with sulfuric acid-hydrogen peroxide mixture solution is mainly used to remove organic matters. However, the use of highly concentrated sulfuric acid-hydrogen peroxide mixture solution of several ten percents requires a lot of reagent (sulfuric acid and hydrogen peroxide) and high-purity water for rinsing after cleaning. This entails not only increased reagent cost for the preparation of sulfuric acid-hydrogen peroxide mixture solution, but also a large scale effluent and waste water treatment system for the used sulfuric acid-hydrogen peroxide mixture solution and high-purity water used in the rinsing process, raising the problems of increased manufacturing cost of products due to installation cost of such treatment system and increased running cost thereof. Cleaning with sulfuric acid-hydrogen peroxide mixture solution usually performed at elevated temperature requires not only some heat source, but also air venting to remove the gas of cleaning agents from the clean room produced by such high temperature cleaning, thus raising a problem of increased cost due to heat source and air venting.
Recently, a method is beginning to be put to practical use which cleans and removes organic matter by using high-purity water with a dissolved ozone gas. It is suggested that cleaning be performed at room temperature for ten minutes by using high-purity water with the dissolved ozone gas of, for example, 2 through 10 ppm. However, this method of using high-purity water with the dissolved ozone gas at room temperature also present a problem in that if organic matter attached is too much or it is scantly degradable, it is difficult to remove organic matter thoroughly, leading to the possibility of leaving organic matter attached even when cleaning for a longer period of time.